This invention relates to heat transfer devices and, more particularly, to thermoelectric heat transfer devices.
Thermoelectric modules are solid-state devices that convert electrical energy to a temperature gradient by using an effect known as the xe2x80x9cPeltier effect.xe2x80x9d Alternatively, thermal energy can be applied to such devices by placing a temperature gradient across them to generate electrical energy due to an effect known as the xe2x80x9cSeebeck effect.xe2x80x9d
When an appropriate electrical voltage is applied, from a battery or other DC source, to a thermoelectric module, one side of the module becomes cold while the other side of the module becomes hot. The xe2x80x9chotxe2x80x9d and xe2x80x9ccoldxe2x80x9d sides of the module depend on the current flow through the device; if the voltage polarity across the module is reversed thereby causing a reverse in the current flow the module xe2x80x9ccoldxe2x80x9d side will become the xe2x80x9chotxe2x80x9d side and vice versa. Thermoelectric modules are typically used by placing them between a heat source and a heat sink, such as a liquid plate, a surface plate, or a convection heat sink. The thermoelectric module will absorb heat on its xe2x80x9ccoldxe2x80x9d side from the heat source and transfer the heat to its xe2x80x9chotxe2x80x9d side and to the heat sink.
A conventional thermoelectric module is composed of an array of thermoelectric elements called xe2x80x9cdicexe2x80x9d which are generally fabricated from Bismuth Telluride and are available in P-types and N-types. The P-type and N-type materials are alloys of Bismuth and Tellurium that have different free electron densities at the same temperature. P-type dice are composed of material having a deficiency of electrons while N-type dice are composed of a material having an excess of electrons. Most modules have an equal number of P-type and N-type dice and one die of each type sharing an electrical interconnection is known as a xe2x80x9ccouple.xe2x80x9d When an electrical current flows through the couple, it attempts to establish a new equilibrium within the materials. The direction of the current will determine if a particular side will cool down or heat up.
The array of P-type and N-type thermoelectric elements are electrically connected in a series chain of couples with alternating element types connected by electrical junctions. When so connected, the electrical junctions form the hot and cold sides of the device with alternating junctions becoming hot and cold, respectively when electrical power is applied to the chain.
In order to form a compact and physically rugged module, the dice are conventionally sandwiched between two ceramic substrates that provide mechanical rigidity and electrical insulation. The P-type and N-type dice are connected electrically in series by electrically conductive materials, usually metal pads, plated on, or attached to, the ceramic substrates. The dice are generally soldered to the pads for mechanical strength.
Such a module is illustrated in FIGS. 1A and 1B, which are cross-sections and top views, respectively, of a thermoelectric module 100. In FIG. 1A, the module 100 is shown sandwiched between a finned convection style heat sink 102 and a heat source 104. In FIG. 1B, the heat sink 102 and the upper substrate 106 have been removed to expose the metal pads that interconnect the thermoelectric elements.
Three such elements 110, 112 and 114 are shown in Figure 1A. As mentioned above, the elements are connected in couples such that P-type dice alternate with N-type dice. For example, thermoelectric elements 110 and 112 are electrically interconnected by electrical pad 116. Similarly, elements 112 and 114 are connected by pad 120. Pads 118 and 122 connect elements 110 and 114 to other elements (not shown.)
FIG. 1B shows a top view that has been exposed to illustrate the electrical interconnections. As shown pads 116, 120, 122, 124, 126, 128, 130 and 132 connect eight thermoelectric elements in series. Element 134 would also be connected by an electrical pad (not shown) to either another element or an electrical power source. This conventional construction is disclosed in several references including the xe2x80x9cCRC Handbook of Thermoelectrics, and Thermoelectric Refrigerationxe2x80x9d by H. J. Goldsmid.
While such devices work well, the efficiency is limited by the conventional construction. The most common type of material used to fabricate substrates 106 and 108 is 96% alumina. This material has poor thermal conductivity for example approximately 35 watts/m xc2x0 C. Since heat, which is transferred from the heat source 104 to the heat sink 102, must pass through two substrates 108 and 106, both of which have poor conductivity, the efficiency of the device is reduced.
Therefore, there is a need for a thermoelectric device with improved thermal efficiency.
In accordance with the principles of the invention, the electrical junctions of either or both sides of a thermoelectric module are placed in direct thermal contact with a heat source or sink or a material to be thermally modified (that is, heated or cooled), thereby eliminating the conventional substrate and its associated thermal resistance. An electrically conductive material such as copper, aluminum or any other known electrical conductor exhibiting relatively high thermal conductivity can be used as the electrical junction between a pair of thermoelectric elements and at the same time function as the transfer medium for the thermal energy produced by the elements.
In one embodiment, the conductive junction passes through a conduit carrying a material to be heated or cooled. In the conduit, the conductive material can be configured into an effective heat transfer shape such as a vane that extends through a non-conducting tube or pipe.
In another embodiment, the geometry of the conductor forming the electrical junction forms a pipe or tube through which material to be heated or cooled is passed.
The use of the inventive module eliminates the need for separate heat exchange devices such as heatsinks, liquid plates, etc., therefore reducing the size of the heat exchanger as well as increasing efficiency by eliminating interfaces between devices.
Additionally this invention can be highly effective in the transfer of waste heat into useable electrical energy. The reduced size and increased efficiency of this design can be effectively used in applications such as automotive exhaust pipes and radiators where the thermoelectric device is built into the apparatus. Many other uses could be considered including steam pipes, process piping, ventilation systems, etc.
In still another embodiment, a protection layer of high thermal conductivity can be applied to the conductive surfaces in order to prevent corrosion or short-circuiting of the device in applications where an electrolytic or ionic fluid is passed by the junction.